Difference between global and device functions

Question

Can anyone describe the differences between __global__ and __device__ ?

When should I use __device__, and when to use __glob

Answer 1

1. __global__ - Runs on the GPU, called from the CPU or the GPU*. Executed with <<>> arguments.
2. __device__ - Runs on the GPU, called from the GPU. Can be used with variabiles too.
3. __host__ - Runs on the CPU, called from the CPU.

*) __global__ functions can be called from other __global__ functions starting
compute capability 3.5.